1. Field of the Invention
The present invention relates to a method of preparing a complex of transition metal oxide and carbon nanotube (CNT) and a complex prepared thereby, and more particularly, to a complex of transition metal oxide and CNT, which can be used as an electrode material for a lithium secondary battery, and a method of preparing the same.
2. Discussion of Related Art
In recent years, according to the development of portable electronic devices, such as mobile phones, laptop computers and camcorders, the demand for compact secondary batteries, such as nickel-hydrogen secondary battery and lithium secondary battery, is increasing. Particularly, the lithium secondary battery using lithium and non-aqueous solvent electrolyte has a high chance to be implemented as a compact, light-weight and high energy density battery, and thus research on this technology is actively progressing.
Generally, the lithium secondary battery is configured using a transition metal oxide such as LiCoO2, LiNiO2 or LiMn2O4 as a cathode material, lithium metal or carbon as an anode material, and an organic solvent containing a lithium ion as an electrolyte between the two electrodes. However, a lithium secondary battery using lithium metal as an anode easily produces crystals on a resin when charging and discharging are repeated and therefore is at high risk for short circuit. Accordingly, a lithium secondary battery using a carbonated or graphitized carbon material as an anode material, and a non-aqueous solvent containing a lithium ion as an electrolyte is practically used.
However, since a carbon-based anode material has high irreversible capacity, initial charge/discharge efficiency and capacity are reduced. Accordingly, in recent years, research on a transition metal oxide as an anode material for a lithium secondary battery has been actively conducted. However, when an electrode is manufactured with a transition metal oxide, carbon particle, such as carbon black used as a conductive material, is difficult to uniformly mix due to the size of the particle itself and is necessarily added in a large amount to enhance electric conductivity. Therefore, the ratio of an electrode active material based on total weight of the electrode material is reduced, and the amount of conductive material not participating in the electrochemical reaction is increased, which serve to decrease capacitance.
In a complex of transition metal oxide and CNT, since the CNT has a quasi-one-dimensional quantum structure, various specific quantum phenomena in low dimension are observed, the CNT has excellent dynamic firmness, chemical stability and thermal conductivity, and the unique characteristics in which the CNT has characteristics of a conductor or semiconductor depends on its structure. When the transition metal oxide is attached to the CNT, excellent material characteristics of the CNT may be improved, and the transition metal oxide may serve as a conductive path. Here, nanotization and distribution control of the transition metal oxides on the CNT are essential to increase the specific surface area of the oxide.
However, a conventional method of preparing a complex of transition metal oxide and CNT essentially includes a step of treating the surface of the CNT with an acid before coating the transition metal oxide. This is because CNT powder is difficult to disperse in a solvent, since the particle surface of the CNT powder that does not undergoing acid treatment is hydrophobic. However, the acid treatment serves as a cause of temporal/economical limitations in terms of practical application and further induces damage to the structure of the CNT at a growing state.
Meanwhile, in recent years, as an anode material for a lithium ion battery, Li4Ti5O12 having a spinel structure has attracted attention, since the material is less altered in volume during charge/discharge and thus has various advantages, such as having a stable lifespan (life cycle) for a long time, preventing the decrease in electrolyte on the surface of the electrode, etc. However, the conventional Li4Ti5O12 having a spinel structure is difficult to nanotize due to limitation in preparation and has poor capacity and rate capability when used as an anode material for a lithium battery. In addition, since it takes a long time (e.g., 24 hours) to synthesize the material, the material has many problems to predetermine to be practically applied to a lithium secondary battery. Therefore, means to solve these problems are required.